My invention aims to eliminate this disadvantage, starting from the consideration that clogging of the perforations of the sleeve is particularly caused by a too rapid outflow of the whey. This accelerated outflow is due to the too great pressure differences in the interior of the column relative to its viscosity. In particular near the upper end of the sleeve where the fluidity of the whey-curd mixture is at its height, a fast outflow of the whey through the perforations may occur, resulting in a clogging of that part of the sleeve.
Clogging of the perforations in the upper region of the sleeve is responsible for the fact that the whey-curd mixture supplied to that region is hardly or not at all thickened, so that the full static pressure of that part of the whey-curd column remains effective. When a certain thickening of this mixture is accomplished in the upper area of the sleeve, the static pressure will decrease, owing to the increased viscosity of the thickened curd. Consequently in case of clogging in the upper region of the sleeve a pressure increase in the lower region of the cylinder becomes noticeable. Due to this pressure increase a clogging of the perforations in that zone will become aggravated. When on the contrary no clogging occurs in said upper region of the sleeve, then the pressure in the lower part of the whey-curd column remains smaller, while in the central and lower part of the sleeve clogging will hardly or not at all happen.